WORMS TO VERTEBRATES 69 



are much less distinct. But in the annelid the mouth 

 is on the second segment ; here it is on the fourth. It 

 has evidently travelled backward. That the mouth of 

 an animal can migrate seems at first impossible, but if 

 we had time to examine the embryology of annelids 

 and insects, it would no longer appear inconceiva- 

 ble or improbable. And its backward migration 

 brought it among the legs which were grasping and 

 chewing the food. And in vertebrates the mouth has 

 changed its position, though not in exactly the same 

 way. Our present mouth is probably not at all the 

 mouth of the primitive ancestor of vertebrates. Thus 

 in the insect three segments have fused around the 

 mouth, and three, possibly four, in front of it. This 

 makes a head worthy of the name. The ganglia 

 of the three post-oral segments, which bear the jaws, 

 have fused in one compound ganglion innervating the 

 mouth and jaws. Those of the three prae-oral seg- 

 ments have fused to form a brain. Eyes are well de- 

 veloped, giving images sometimes accurate in detail, 

 sometimes very rude. Ears are not uncommon. The 

 sense of smell is often keen. 



Perhaps the greatest advance of the insect is its 

 adaptation to land life. This gives it a larger supply 

 of oxygen than any aquatic animal could ever obtain. 

 This itself stimulates every function, and all the work 

 of the body goes on more energetically. Then the 

 heat produced is conducted off far less rapidly than 

 in aquatic forms. Water is a good conductor of heat, 

 and nearly all aquatic animals are cold-blooded. The 

 few which are warm-blooded are protected by a thick 

 layer of non-conducting fat. In all land animals, 

 even when cold-blooded, the work of the different sys- 



